ASoC:: remove a redundant snd_soc_unregister_codec call in wm8988_register

snd_soc_unregister_codec is called twice if snd_soc_register_dai fail.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>